Sputtering apparatus

ABSTRACT

SPUTTER APPARATUS OF THE ELECTRON ASSISTED TYPE WITH A &#34;FOUR-ELECTRODE&#34; SPUTTER STRUCTURE AND SUBSTRATE ENCLOSING AN ELONGATED SPACE. AN ELONGATED ELECTRON SOURCE EXTENDS ALONG THE FULL LENGTH OF AN EDGE OF THE SPACE AND AN ELONGATED ANODE EXTENDS ALONG AN OPPOSITE EDGE. AN ELONGATED GRID ADJACENT THE ELECTRON EMITTER MAINTAINS THE DISCHARGE AND MASKS THE EMITTER. THE ARRANGEMENT ALLOWS STABLE LOW PRESSURE SPUTTERING AT HIGH RATES AND WITH HIGH UNIFORMITY WITHOUT USE OF AN ASSISTING MAGNETIC FIELD AND AFFORDS GREATER FREEDOM FOR EFFECTIVE DESIGN OF THE ENCLOSING VACUUM SYSTEM.

W M OR m N P O A RG Am R J E -T Am P 5 June 8, 1971 5 Sheets-Sheet 1 Filed Dec. 18, 1968 INVENTOR.

BY QRNQ A. Ram-50) ATTORNEY June 8, 1971 J, oNso 3,53,89

SPUTTERING APPARATUS Filed Dec. 18, 1968 5 Sheets-Sheet June & 1971 A. J. ARONSON 58 SPUTTERING APPARATUS Filed Dec. 18, 1968 5 Sheets-Sheet 5 June 8, 1971 A. J. ARONSON 3,5833

SPUTTERING APPARATUS Filed Dec. 18, 1968 5 Sheets-Sheet 4 United States Patent 3,583,899 SPUTTERING APPARATUS Arnold J. Aronson, Brookline, Mass., assignor to Norton I Company, Worcester, Mass.

Filed Dec. 18, 1968, Ser. No. 784,581 Int. Cl. C23c /00 US. Cl. 204298 13 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to sputtering apparatus and particularly to sputter coating apparatus. Such apparatus is well known in the art. Particular forms of such apparatus include diode sputtering units and electron assisted sputtering apparatus. The latter which are available in triode and tetrode configurations afford the possibility of operation at lower pressures than diode units with improved purity and adherence of coatings.

One limitation of prior art electron assisted units is that they require an assisting magnetic field for certain commercial operations. This imposes some constraints on effective design of the sputtering apparatus and the surrounding vacuum system. Another problem ofsuch apparatus is that uniformity of coating (consistent with high rate of coating) of prior art electron assisted sputter coaters 'has been less than desired. Because of the lack of desired uniformity, rotation and multi-layer coating of substrates is practiced with dangers of interlayer contamination or faults due to temperature changes between coating steps. A related problem of electron assisted machines is the ditficulty of adapting them to mass production use in view of the above limitations.

It is therefore the principal object of the invention to overcome the foregoing problems in electron-assisted sput ter coating apparatus and provide an improved apparatus of this class which provides sufficient cooling uniformity to allow single pass coating and stable operation without the necessity for magnetic field assistance and further provides a high degree of mass production capability consistent with economy of manufacture and case of operation by semi-skilled personnel. It is a further object of the invention to provide for coating of multiple coating materials and/ or multiple coated substrates without sacrifice of purity or reliability of the coating process. It is a further object of the invention to provide a good vacuum pumping arrangement of the apparatus as a whole consistent with the foregoing objects.

In general, the objects of the invention are achieved through a sputtering apparatus in which a plasma is elongated cylindrical shield surrounding the emitter and having an elongated opening for electron exit. Associated with the emitter is an elongated auxiliary anode, or grid, which serves to start and maintain the sputtering discharge at low voltages and comprises an extension reaching into the opening of the shield turret, but electrically isolated therefrom.

A variation of the apparatus provides an electron emitter and associated grid and shield at each of the elongated edges with each grid acting as a primary anode for the remote emitter. This arrangement gives further improvement in coating uniformity.

The target and substrate holder oppose each other straddling the plasma when used for sputter coating and are arranged in vertical planes to avoid dirt pickup. Preferably the target and/or substrate holder comprise rotatable turrets for multiple target sputtering in sequence and/or increased substrate handling capacity. Shielding arrangements are provided to prevent stray sputtering or coating of the target or substrate faces not in the plasma straddling coating position.

The above described structure is housed in a vacuum chamber of straight cylinder design, vertically arranged, with a full area top opening sealed during operation by a dome cover. Substrate turrets can be inserted or removed through the top opening. Pressure and gas flow control is provided by an inert gas inlet into the electron emitter shield and a vacuum pumping port extending out of the side wall of the chamber behind the target. As a Whole this arrangement provides more effective design for purposes of structure conveient height and gas load and conductance than most prior art arrangments and is less vulnerable to foreign objects falling into the pumping port. This improved chamber and system design is made feasible by the above described internal apparatus of the sputtering unit.

The invention also'avoids the need for a coiled filament electron emitter of prior art apparatus and inherently obtains equivalent emission length in the above described apparatus using a straight line form of filament.

Other objects, features and advantages of the invention will in part be obvious from the general description given thus far and from the following description of specific embodiments of the invention, described in connection with the accompanying drawings wherein:

FIGS. 12 are schematic side and top views of the improved sputtering apparatus according to a first, and preferred, embodiment of the invention. FIG. 2A is a cross section view of the electron emitter, grid and shield, and FIG. 3 is an isometric sketch of the sputtering and coating parts.

FIGS. 4 and 5 are side and top views, similar to FIGS. 1 and 2, for a second apparatus embodiment.

FIGS. 6-8 are outlines of coating thickness measurements resulting from use of the apparatus.

FIGS. 1-3

Referring to FIG. 1 first, the sputtering apparatus is contained in a cylindrical hermetically sealed chamber 10 which has a door 12 demountably sealed to the chamber by a seal 14. The door covers the full diameter of the cylindrical chamber and its arrangement atop the vertical cylinder facilitates loading and unloading of substrates and for maintenance between operations under vacuum. The chamber is pmped, via an exit port 16 and a butterfly throttle valve 18, by a diffusion pump 20 backed by a mechanical coughing pump 22. The location of the exit port 16 is, preferably, as shown in FIG. 2. Its location is roated degrees in FIG. 1 for purpose of illustration. Additional conventional gas pressure control means such as pumps, valves, pressure measuring gauges and inert gas inlet are provided but not shown in the drawings. A bottom plate 24 of the chamber contains electrical insulating feedthroughs 26 for the various electrical conductors 28 leading to the sputter electrodes. The conductors have the form of copper pipes and coolant water is passed through these pipes.

Within the chamber is a tetrode sputtering arrangement. The four electrodes of the tetrode are an elongated filament electron emitter 52, a primary anode 54, an auxiliary anode and sputter target 58.

Referring now to FIG. 2 (which is a cross-section view from above of the sputter electrode structure) as well as FIG. 1, there is shown a substrate holder 60 which mounts several substrates 62 which are to be coated by material sputtered from the front face 581 of target 58. The sputter target 58 is of planar form and has two elongated side edges 582, 583. The target and substrate holder define between them a thin plasma confining zone 70 which is enclosed on its thin sides by the plasma forming structureanodes 54 and 56 and emitter 52 which extend along essentially the full length of the elongated edges 582, 583 of target 58. Optionally, top and bottom grounded metal shields may be added to further define the plasma zone; but this is not necessary where the cover 12 and base plate 24 are grounded metal members and over half the height between them is occupied by the electrode structure. An elongated annular metal shield 521 at ground potential surrounds the electron emitter 52; the shield provides electrostatic focusing of the electrons emitted from the emitter towards an opening 522 in the shield.

FIG. 2A shows the electron emitter 52 in cross-section, surrounded by shield 521 which has an elongated opening 522 facing towards the plasma region 70 (see also FIGS. 1-2). Grid electrode 56 is located adjacent the opening and comprises two extension plates 561 extending to and through the opening 522 to effectively start and maintain an electrical discharge plasma, in conjunction with emitter 52. This arrangement also provides effective masking of emitter 52 from the substrate and target. The emitter typically has the form of a braid or twist of 3 tungsten wires of 30 mil diameter or an equivalent ribbon. A helically coiled emitter is not necessary because of the great length of emitter 52 and not desirable because of the greater difliculty of masking a helix.

FIG. 3 is an isometric sketch of the apparatus showing particularly the high volume production capacity and flexibility of the apparatus for semiconductor wafer substrates are mounted (via mounting pins, not shown) on each substrate holder 60 of the eight-sided multi-holder turret. Different targets 580, 581 can be provided for putting down multiple coatings on each substrate. The targets are rotated within a grounded shield 58 to essentially limit sputtering to the target facing the plasma zone 70. Further protection can be afforded by providing separate electrical feedthroughs to the two targets 580, 581 so that target voltage is not applied to the inactive target.

Referring to all of FIGS. 1-3, it can be seen that the grid comprises an elongated loop of coolant pipe 56 with plate extensions 561 extending to and through the opening 522 of cathode turret 521.

Small shields 602 at the edge of each face of the substrate holder turret and additional grounded shields 603 (shown in FIG. 1 only) prevent sputtering of target materials on substrates not directly facing the target.

The substrate holder turret 60 typically has eight four inch by six inch sides (carrying six two inch diameter wafers on mounting pins) and is inserted and removed through the top opening of chamber 10 between production runs. It is clamped down to a plate 604 which is cooled by water fed via rotary feedthrough 604. Thus the substrates and substrate holders are cooled during sputter coating for control of plasma and coating process.

FIGS 4-5 Referring now to FIGS. 4-5, a second embodiment of the apparatus is shown in two figures corresponding to FIGS. 1 and 2 of the first embodiment. In this second embodiment, two elongated electron emitters 52 and 52' are provided along two long edges 582, 583 of target face 581. Associated with each electron emitter is a surrounding grounded turret (521, 521) with an opening (522, 522') and a long grid electrode (56, 56') with plate extensions (561, 561') extending to and through the turret opening. Each grid acts as a primary anode for the far electron emitter and as auxiliary anode for the near electron emitter.

The arrangement provides greater coating uniformity at lower coating rates compared to the apparatus of FIGS. 13. The interconnected grids 56 and 56' cause the apparatus as a whole to act as a full wave rectifier. The same result is accomplished with soft reactance controlled DC. voltage supplies connected to grids 56, 56'.

EXAMPLES Example 1 PROCEDURE Time, hours/ minutes tep 00:00-00:03 Release system, under vacuum from prior production,

by admitting an atmosphere of argon.

Lift open the cover 12.

Unlock and lift out the substrate holder.

Remove waters 62.

Reload with next batch of waters. [It a spare holder is stocked, it can be preloaded, eliminating the time of the previous two step] 00:11-00:12 Replace the substrate holder into the system and lock in. Close the hinged cover. Pump the system to 10 torr range. Adjust the filament to 13 volts, amperes A.C.

Backfill with argon and repump. (In some cases, two

back fillings may be desired.) 00:34-00:35 Turn the anode and grid supplies on by means of the 11.0. line switch (they are normally preset). Immediately turn on the high voltage. (This can also be preset!) 00:35-00:55 Sputter titanium at A./min. to get 3,000 angstroms.

00:55-03:18 Index the substrate holder turret to the next face 60.

Sputter 20 mins. Repeat for faces 3 to 8.

03:18-03:19 Turn off the high voltage and rotate the target to the other face.

03:19-03:29 Turn the high voltage on and sputter platinum {or 8 mins. at 275 A./min. to get 3,000 angstroms.

03:29-04:25 Index the substrate holder turret to next face. Sputter 8 mins. Repeat for faces 3 to 8.

04:25-04:26 Turn off power supplies and filament. Return to the top of this page for the next run.

. 1 In some applications, a sputter cleaning of the target face would be included. In many cases the releasing by Argon may eliminate this need. (If done, allow 10 minutes.)

Some non-limiting examples of operation of the equipment for coating thickness measurement are now given.

Example 2 Sputter coating apparatus as shown in FIGS. 1-3 was operated to coat one substrate holder face 60 covered with a 4 inch by 6 inch flat glass (made by Pilkington float process) with stainless steel. Seven 30 mil tungsten wires, evenly spaced and horizontally arranged were held across the glass face to produce coating steps (by masking) for interferometry measurements of coating thickness. The apparatus was operated with the target at 2000 volts and milliamperes AC. and the primary anode drawing 7 amperes DC. at 70 volts with the auxiliary anode drawing 2 amperes DC. at 40 volts. The emitter 52 was operated at 13 volts, 88 amperes AC. The operation was carried out for 30 minutes with argon flowing in at 22.4 cc. per minute and total pressure maintained at about 2-3 microns by the pumping system.

After the coating the thickness of the coating was measured at various points on the glass. The measurements are given in FIG. 6 and are in hundreds of angstroms.

Example 3 A test similar to Example 2 was carried out with only the following variations in conditions: 8 amperes primary anode current, 170 milliamperes target current and a total time of 37 minutes. The coating thickness results are indicated in FIG. 7.

Example 4 A similar test was carried out with dual emitter apparatus as in FIGS. 4-5 with the following variations of conditions from Example 2: each grid (52, 52 at 7 amperes D.C., each electron emitter at 13 volts, 85 amperes, target current at 142 milliamperes. The coating thickness results are given in FIG. 8.

It can be seen from these examples that high uniformity of coating is attained by the apparatus of the present invention.

Several variations can be made within the scope of the present invention. For instance high voltage can be applied to the substrates for sputter etching or bias sputtering coating. Substrate and/or target turret rotation can be automated with use of a timer or coating rate monitor to signal rotation steps. Substrate turret rotation can be continuous if multi-pass coating is tolerable. The target may be triangular or circular, rather than rectangular as shown in the examples. For purposes of a circular target, the elongated edge is any 120 degrees of are along the target edge. The term adjacent as used herein is meant in contrast to the very remote spacings of a prior art apparatus, but for purposes of definition, as necessary, may be taken as equal to or less than half the length of the elongated edge of the target. A movable shutter may be provided to cover the substrate holder adjacent the plasma to allow sputter cleaning of the target, the shutter can then be removed to allow coating. Some departures can be made from the preferred features shown, e.g. horizontally arranged substrate turret, bottom of the chamber pumping port to surrender some advantages of the invention while retaining others. The elongated electron emitter may be made up of a string of shorter emitters or center tapped to provide for variation of emission current along the total length for finer tailoring of coating uniformity.

Still other variations of the present invention will be apparent to those skilled in the art once given the benefit of the present disclosure. Accordingly, it is intended that the above disclosure shall be read as illustrative and not in a limiting sense.

What is claimed is:

1. An improved sputtering apparatus comprising:

(a) means forming a vacuum chamber,

(b) means forming a sputter target, of essentially planar form with at least two elongated edges, disposed within said chamber,

(c) plasma forming means disposed within said chamber adjacent at least two of said elongated edges of the sputter target and comprising an elongated electron emitter constituted by an elongated filament extending parallel to the target and extending along at least half the full length of one of said elongated edges, and further comprising a similarly elongated primary anode electrode located along a second edge atmospheric gas pressure within the chamber suitable for sputtering.

2. The apparatus of claim 1 wherein the apparatus comprises a coating substrate holder disposed in opposing relation to said target.

3. The apparatus of claim 2 wherein the second member comprises a rotatable multiple plane substrate holder turret.

4. The apparatus of claim 3 wherein the chamber is a straight cylinder tank vertically arranged and having a top cover door.

5. The apparatus of claim 2 wherein the sputter target and substrate holder are arranged in substantially vertical planes.

6. The apparatus of claim 1 wherein the plasma forming means (c) comprises electrostatic focussing means in the form of an elongated annular shield surrounding the emitter, said shield having an elongated opening facing into the region in front of the target, a primary anode located at a second elongated edge of said target, an auxiliary anode located between said shield opening and the target edge and having an extension thereof extending through said shield opening and into said shield without touching the shield to define an elongated plasma extraction passage with a passage forming wall at auxiliary anode potential.

7. The apparatus of claim 6 wherein the auxiliary anode has the form of an elongated loop electrode essentially paralleling the contour of the shield opening and the extension has the form of a pair of parallel plates extending from the long legs of the auxiliary anode loop to and through said elongated opening.

8. The apparatus of claim 1 comprising two of said elongated electron emitters with local auxiliary anodes located along first and second elongated edges of the target with each of said auxiliary anodes acting as a primary anode for the more remote electron emitter.

9. The apparatus of claim 1 wherein the electrical bias means (d) is constructed and arranged to provide a cyclically alternating voltage supply to the respective auxiliary anodes of the two plasma sources.

10. The apparatus of claim 1 wherein the plasma forming means are constructed and arranged to optically mask said electron emitter from said target.

11. The apparatus of claim 1 wherein said means (a) comprises a rotable multiple target turret.

12. The apparatus of claim 1 wherein two of said elongated electron emitters are provided along two of said elongated target edges in essentially opposing relationship and wherein the bias and energizing means apply a cyclical alternating injection of plasma from the respective emitters.

13. The apparatus of claim 1 wherein the chamber is a straight cylinder, vertically arranged and having a top cover door, and further comprising a pumping port in the side wall of the chamber and vacuum pump means connected to the port.

References Cited UNITED STATES PATENTS 3,324,019 6/1967 Laegreid et a1 204298 3,400,066 9/1968 Caswell et a1. 204-192 3,451,917 6/1969 Mosesow 204298 3,464,907 9/1969 Froemel et al 204-298 3,481,854 12/1969 Lane 204--l92 3,507,774 4/1970 Muly 204298 TA-HSUNG TUNG, Primary Examiner S. S. KANTER, Assistant Examiner U.S. Cl. X.R. 204-1 92 

